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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPsec Working Group R. Housley 3 Internet Draft Vigil Security 4 expires in six months February 2003 6 Using AES CCM Mode With IPsec ESP 7 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with all 12 provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering Task 15 Force (IETF), its areas, and its working groups. Note that other 16 groups may also distribute working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 This document is a submission to the IETF Internet Protocol Security 30 (IPsec) Working Group. Please send comments on this document to the 31 working group mailing list (ipsec@lists.tislabs.com). 33 Distribution of this memo is unlimited. 35 Abstract 37 This document describes the use of AES CCM Mode, with an explicit 38 initialization vector, as an IPsec Encapsulating Security Payload 39 (ESP) mechanism to provide confidentiality, data origin 40 authentication, connectionless integrity. 42 1. Introduction 44 The Advanced Encryption Standard (AES) [AES] is a block cipher, and it 45 can be used in many different modes. This document describes the use 46 of AES in CCM (Counter with CBC-MAC) mode (AES-CCM), with an explicit 47 initialization vector (IV), as an IPsec Encapsulating Security Payload 48 (ESP) [ESP] mechanism to provide confidentiality, data origin 49 authentication, connectionless integrity. 51 This document does not provide an overview of IPsec. However, 52 information about how the various components of IPsec and the way in 53 which they collectively provide security services is available in 54 [ARCH] and [ROAD]. 56 1.1. Conventions Used In This Document 58 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 59 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 60 document are to be interpreted as described in [STDWORDS]. 62 2. AES-CCM Mode 64 CCM is a generic authenticate-and-encrypt block cipher mode [CCM]. In 65 this specification, CCM is used with the AES [AES] block cipher. 67 AES-CCM has two parameters: 69 M M indicates the size of the integrity check value (ICV). 70 CCM defines values of 4, 6, 8, 10, 12, 14, and 16 octets; 71 However, to maintain alignment and provide adequate 72 security, only the values that are a multiple of four and 73 are at least eight are permitted. Implementations MUST 74 support M values of 8 octets and 16 octets, and 75 implementations MAY support an M value of 12 octets. 77 L L indicates the size of the length field in octets. CCM 78 defines values of L between 2 octets and 8 octets. 79 Implementations MUST support an L value of 4 octets, which 80 accommodates a full Jumbogram [JUMBO]; however, the length 81 includes all of the encrypted data, which also includes 82 the ESP Padding, Pad Length, and Next Header fields. 84 There are four inputs to CCM originator processing: 86 key 87 A single key is used to calculate the ICV using CBC-MAC and to 88 perform payload encryption using counter mode. AES supports 89 key sizes of 128 bits, 192 bits, and 256 bits. The default key 90 size is 128 bits, and implementations MUST support this key 91 size. Implementations MAY also support key sizes of 192 bits 92 and 256 bits. 94 nonce 95 The size of the nonce depends on the value selected for the 96 parameter L. It is 15-L octets. Implementations MUST support 97 a nonce of 11 octets. The construction of the nonce is 98 described in section 4. 100 payload 101 The payload of the ESP packet. The payload MUST NOT be longer 102 than 4,294,967,295 octets, which is the maximum size of a 103 Jumbogram [JUMBO]; however, the ESP Padding, Pad Length, and 104 Next Header fields are also part of the payload. 106 AAD 107 CCM provides data integrity and data origin authentication for 108 some data outside the payload. CCM does not allow additional 109 authenticated data (AAD) to be longer than 110 18,446,744,073,709,551,615 octets. The ICV is computed from 111 the ESP header, Payload, and ESP trailer fields, which is 112 significantly smaller than the CCM imposed limit. The 113 construction of the AAD described in section 5. 115 AES-CCM requires the encryptor to generate a unique per-packet value, 116 and communicate this value to the decryptor. This per-packet value 117 is one of the component parts of the nonce, and it is referred to as 118 the initialization vector (IV). The same IV and key combination MUST 119 NOT be used more than once. The encryptor can generate the IV in any 120 manner that ensures uniqueness. Common approaches to IV generation 121 include incrementing a counter for each packet and linear feedback 122 shift registers (LFSRs). 124 AES-CCM employs counter mode for encryption. As with any stream 125 cipher, reuse of the IV same value with the same key is catastrophic. 126 An IV collision immediately leaks information about the plaintext in 127 both packets. For this reason, it is inappropriate to use this CCM 128 with statically configured keys. Extraordinary measures would be 129 needed to prevent reuse of an IV value with the static key across 130 power cycles. To be safe, implementations MUST use fresh keys with 131 AES-CCM. The Internet Key Exchange (IKE) [IKE] protocol can be used 132 to establish fresh keys. 134 3. ESP Payload 136 The ESP payload is comprised of the IV followed by the ciphertext. 137 The payload field, as defined in [ESP], is structured as shown in 138 Figure 1. 140 0 1 2 3 141 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 142 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 143 | Initialization Vector | 144 | (8 octets) | 145 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 146 | | 147 ~ Encrypted Payload (variable) ~ 148 | | 149 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 150 | | 151 ~ Authentication Data (variable) ~ 152 | | 153 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 155 Figure 1. ESP Payload Encrypted with AES-CCM 157 3.1. Initialization Vector (IV) 159 The AES-CCM IV field MUST be eight octets. The IV MUST be chosen by 160 the encryptor in a manner that ensures that the same IV value is used 161 only once for a given key. The encryptor can generate the IV in any 162 manner that ensures uniqueness. Common approaches to IV generation 163 include incrementing a counter for each packet and linear feedback 164 shift registers (LFSRs). 166 Including the IV in each packet ensures that the decryptor can 167 generate the key stream needed for decryption, even when some 168 datagrams are lost or reordered. 170 3.2. Encrypted Payload 172 The encrypted payload contains the ciphertext. 174 AES-CCM mode does not require plaintext padding. However, ESP does 175 require padding to 32-bit word-align the authentication data. The 176 Padding, Pad Length, and Next Header fields MUST be concatenated with 177 the plaintext before performing encryption, as described in [ESP]. 179 3.3. Authentication Data 181 AES-CCM provides an encrypted ICV. The ICV provided by CCM is 182 carried in the Authentication Data fields without further encryption. 183 Implementations MUST support ICV sizes of 8 octets and 16 octets. 184 Implementations MAY also support ICV 12 octets. 186 4. Nonce Format 188 Each packet conveys the IV that is necessary to construct the 189 sequence of counter blocks used by counter mode to generate the key 190 stream. The AES counter block 16 octets. One octet is used for the 191 CCM Flags, and 4 octets are used for the block counter, as specified 192 by the CCM L parameter. The remaining octets are the nonce. These 193 octets occupy the second through the twelfth octets in the counter 194 block. Figure 2 shows the format of the nonce. 196 0 1 2 3 197 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 198 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 199 | Salt | 200 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 201 | Initialization Vector | 202 | | 203 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 205 Figure 2. Nonce Format 207 The components of the nonce are as follows: 209 Salt 210 The salt field is 24 bits. As the name implies, it contains an 211 unpredictable value. It MUST be assigned at the beginning of 212 the security association. The salt value need not be secret, 213 but it MUST NOT be predictable prior to the beginning of the 214 security association. 216 Initialization Vector 217 The IV field is 64 bits. As described in section 3.1, the IV 218 MUST be chosen by the encryptor in a manner that ensures that 219 the same IV value is used only once for a given key. 221 This construction permits each packet to consist of up to: 223 2^32 blocks = 4,294,967,296 blocks 224 = 68,719,476,736 octets 226 This construction provides more key stream for each packet than is 227 needed to handle any IPv6 Jumbogram [JUMBO]. 229 4. AAD Construction 231 The data integrity and data origin authentication for the SPI and 232 (Extended) Sequence Number fields is provided without encrypting 233 them. Two formats are defined: one for 32-bit sequence numbers and 234 one for 64-bit extended sequence numbers. The format with 32-bit 235 sequence numbers is shown in Figure 3, and the format with 64-bit 236 extended sequence numbers is shown in Figure 4. 238 0 1 2 3 239 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 240 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 241 | SPI | 242 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 243 | 32-bit Sequence Number | 244 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 246 Figure 3. AAD Format with 32-bit Sequence Number 248 0 1 2 3 249 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 | SPI | 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | 64-bit Extended Sequence Number | 254 | | 255 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 257 Figure 4. AAD Format with 64-bit Extended Sequence Number 259 5. Packet Expansion 261 The initialization vector (IV) and the integrity check value (ICV) is 262 the only sources of packet expansion. The IV always adds 8 octets to 263 the front of the payload. The ICV is added at the end of the 264 payload, and the CCM parameter M determines the size of the ICV. 265 Implementations MUST support M values of 8 octets and 16 octets, and 266 implementations MAY also support an M value of 12 octets. 268 6. IKE Conventions 270 As previously described, implementations MUST use fresh keys with 271 AES-CCM. The Internet Key Exchange (IKE) [IKE] protocol can be used 272 to establish fresh keys. This section describes the conventions for 273 obtaining the unpredictable salt value for use in the nonce from IKE. 274 Note that this convention provides a salt value that is secret as 275 well as unpredictable. 277 IKE makes use of a pseudo-random function (PRF) to derive keying 278 material. The PRF is used iteratively to derive keying material of 279 arbitrary size. Keying material is extracted from the output string 280 without regard to boundaries. 282 IKE uses the PRF to generate an output stream that parsed into five 283 keys: SK_d, SK_ai, SK_ar, SK_ei, and SK_er. SK_d is used to derive 284 new keys for the child security associations. SK_ai and SK_ar are 285 the authentication keys for the initiator and the responder, 286 respectively. SK_ei and SK_er are the encryption keys for the 287 initiator and the responder, respectively. 289 SK_ai and SK_ei are used to protect traffic from the initiator to the 290 responder. SK_ar and SK_er are used to protect traffic from the 291 responder to the initiator. 293 The size of SK_ei and SK_er are each three octets longer than is 294 needed by the associated AES key. The keying material is used as 295 follows: 297 AES-CCM with a 128 bit key 298 SK_ei and SK_er are each 19 octets. The first 16 octets are 299 the 128-bit AES key, and the remaining three octets are used as 300 the salt value in the counter block. 302 AES-CCM with a 192 bit key 303 SK_ei and SK_er are each 27 octets. The first 24 octets are 304 the 192-bit AES key, and the remaining three octets are used as 305 the salt value in the counter block. 307 AES-CCM with a 256 bit key 308 SK_ei and SK_er are each 35 octets. The first 32 octets are 309 the 256-bit AES key, and the remaining three octets are used as 310 the nonce value in the counter block. 312 7. Test Vectors 314 To be supplied. 316 8. Security Considerations 318 AES-CCM employs counter (CTR) mode for confidentiality. If a counter 319 value is ever used for more that one packet with the same key, then 320 the same key stream will be used to encrypt both packets, and the 321 confidentiality guarantees are voided. 323 What happens if the encryptor XORs the same key stream with two 324 different packet plaintexts? Suppose two packets are defined by two 325 plaintext byte sequences P1, P2, P3 and Q1, Q2, Q3, then both are 326 encrypted with key stream K1, K2, K3. The two corresponding 327 ciphertexts are: 329 (P1 XOR K1), (P2 XOR K2), (P3 XOR K3) 331 (Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3) 333 If both of these two ciphertext streams are exposed to an attacker, 334 then a catastrophic failure of confidentiality results, since: 336 (P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1 337 (P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2 338 (P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3 340 Once the attacker obtains the two plaintexts XORed together, it is 341 relatively straightforward to separate them. Thus, using any stream 342 cipher, including AES-CTR, to encrypt two plaintexts under the same 343 key stream leaks the plaintext. 345 Therefore, AES-CCM should not be used with statically configured 346 keys. Extraordinary measures would be needed to prevent the reuse of 347 a counter block value with the static key across power cycles. To be 348 safe, implementations MUST use fresh keys with AES-CCM. The Internet 349 Key Exchange (IKE) [IKE] protocol can be used to establish fresh 350 keys. 352 When IKE is used to establish fresh keys between two peer entities, 353 separate keys are established for the two traffic flows. If a 354 different mechanism is used to establish fresh keys, one that 355 establishes only a single key to encrypt packets, then there is a 356 high probability that the peers will select the same IV values for 357 some packets. Thus, to avoid counter block collisions, ESP 358 implementations that permit use of the same key for encrypting and 359 decrypting packets with the same peer MUST ensure that the two peers 360 assign different salt values to the security association (SA). 362 AES with a 128-bit key is vulnerable to the birthday attack after 363 2^64 blocks are encrypted with a single key, regardless of the mode 364 used. Since ESP with Extended Sequence Numbers allows for up to 2^64 365 packets in a single security association (SA), there is real 366 potential for more than 2^64 blocks to be encrypted with one key. 367 Implementations SHOULD generate a fresh key before 2^64 blocks are 368 encrypted with the same key, or implementations SHOULD make use of 369 the longer AES key sizes. Note that ESP with 32-bit Sequence Numbers 370 will not exceed 2^64 blocks even if all of the packets are maximum- 371 length Jumbograms. 373 9. Design Rationale 375 In the development of this specification, the use of the ESP sequence 376 number field instead of an explicit IV field was considered. This 377 section documents the rationale for the selection of an explicit IV. 378 This selection is not a cryptographic security issue, as either 379 approach will prevent counter block collisions. 381 The use of the explicit IV does not dictate the manner that the 382 encryptor uses to assign the per-packet value in the counter block. 383 This is desirable for several reasons. 385 1. Only the encryptor can ensure that the value is not used for 386 more than one packet, so there is no advantage to selecting a 387 mechanism that allows the decryptor to determine whether counter 388 block values collide. Damage from the collision is done, whether 389 the decryptor detects it or not. 391 2. The use of explicit IVs allows adders, LFSRs, and any other 392 technique that meets the time budget of the encryptor, so long as 393 the technique results in a unique value for each packet. Adders 394 are simple and straightforward to implement, but due to carries, 395 they do not execute in constant time. LSFRs offer an alternative 396 that executes in constant time. 398 3. Complexity is in control of the implementer. Further, the 399 decision made by the implementer of the encryptor does not make 400 the decryptor more (or less) complex. 402 4. The assignment of the per-packet counter block value needs to 403 be inside the assurance boundary. Some implementations assign the 404 sequence number inside the assurance boundary, but others do not. 405 A sequence number collision does not have the dire consequences, 406 but, as described in section 6, a collision in counter block 407 values has disastrous consequences. 409 5. Using the sequence number as the IV is possible in those 410 architectures where the sequence number assignment is performed 411 within the assurance boundary. In this situation, the sequence 412 number and the IV field will contain the same value. 414 6. By decoupling the IV and the sequence number, architectures 415 where the sequence number assignment is performed outside the 416 assurance boundary are accommodated. 418 The use of an explicit IV field directly follows from the decoupling 419 of the sequence number and the per-packet counter block value. The 420 additional overhead (64 bits for the IV field) is acceptable. This 421 overhead is significantly less overhead associated with Cipher Block 422 Chaining (CBC) mode. As normally employed, CBC requires a full block 423 for the IV and, on average, half of a block for padding. AES-CCM 424 confidentiality processing with an explicit IV has about one-third of 425 the overhead as AES-CBC, and the overhead is constant for each 426 packet. 428 10. IANA Considerations 430 IANA has assigned nine ESP transform numbers for use with AES-CCM 431 with an explicit IV: 433 for AES-CCM with a 128 bit AES key and an 8 octet ICV; 434 for AES-CCM with a 192 bit AES key and a 12 octet ICV; 435 for AES-CCM with a 256 bit AES key and a 16 octet ICV; 436 for AES-CCM with a 128 bit AES key and an 8 octet ICV; 437 for AES-CCM with a 192 bit AES key and a 12 octet ICV; 438 for AES-CCM with a 256 bit AES key and a 16 octet ICV; 439 for AES-CCM with a 128 bit AES key and an 8 octet ICV; 440 for AES-CCM with a 192 bit AES key and a 12 octet ICV; and 441 for AES-CCM with a 256 bit AES key and a 16 octet ICV. 443 11. Acknowledgements 445 Doug Whiting and Niels Ferguson worked with me to develop CCM mode. 446 We developed CCM mode as part of the IEEE 802.11i security effort. 447 One of the most attractive aspects of CCM mode is that it is 448 unencumbered by patents. I acknowledge the companies that supported 449 the development of an unencumbered authenticated encryption mode (in 450 alphabetical order): 452 Hifn 453 Intersil 454 MacFergus 455 RSA Security 457 12. References 459 This section provides normative and informative references. 461 12.1. Normative References 463 [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard 464 (AES)," November 2001. 466 [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)," 467 Work In Progress. . 469 [CCM] Whiting, D., Housley, R., and N. Ferguson, 470 "Counter with CBC-MAC (CCM)," Work In Progress. 471 . 473 [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate 474 Requirement Levels," RFC 2119, March 1997. 476 12.2. Informative References 478 [ARCH] Kent, S. and R. Atkinson, "Security Architecture for 479 the Internet Protocol," RFC 2401, November 1998. 481 [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange 482 (IKE)," RFC 2409, November 1998. 484 [ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security 485 Document Roadmap," RFC 2411, November 1998. 487 13. Author's Address 489 Russell Housley 490 Vigil Security, LLC 491 918 Spring Knoll Drive 492 Herndon, VA 20170 493 USA 494 housley@vigilsec.com 496 Full Copyright Statement 498 Copyright (C) The Internet Society 2003. 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